1. Field of the Invention
The present invention relates to an amplifier circuit, and more particularly, to a technique for controlling the output power of a high-frequency amplifier circuit.
2. Description of the Background Arts
In an amplifier circuit employed in the transmitter output stage of a transceiver or the like, a high-frequency signal is amplified with a multistage amplifier, whereby a high power output signal is obtained. However, when the distance from the transceiver to the opposite transceiver of the radio base station is relatively short, for example, the communication with the opposite transceiver or the radio station can be attained even with a relatively low power radio signal. Therefore, an amplifier circuit of variable output power type is also developed and used in the transceiver, in which the output power can be switched between high power and low power.
FIG. 10 shows a conventional amplifier circuit of the variable output power type, which is provided in the output stage of a transceiver. The amplifier circuit is provided with a multistage amplifier 1 having a plurality of unit amplifiers 1a-1c connected in series. A high-frequency input signal SI supplied to an input terminal 3 is amplified with the multistage amplifier 1 thereby to become a high-frequency amplified output signal SO. The high-frequency signal SO is delivered to a load circuit 4 through an output terminal 3. DC powers for driving the unit amplifiers 1a-1c is supplied from a DC power source 5 through power input terminals 6a-6c, respectively, where an electronic variable resistor 7 is inserted between the power source 5 and the initial stage unit amplifier 1a. The electronic variable resistor 7 is so constructed that the equivalent internal resistance thereof is varied according to the level of a resistance control signal S.sub.R.
When it is intended that the power of the output signal SO is varied, the equivalent internal resistance of the electronic variable resistor 7 is changed by changing the level of the resistance control signal S.sub.R. Accordingly, the DC bias power supplied to the initial stage unit amplifier 1a is changed, and the gain in the initial stage unit amplifier 1a is changed. As a result, the total gain in the multistage amplifier 1 is changed and the power of the output signal SO supplied to the load circuit 4 through the output terminal 3 is changed.
Each of the unit amplifiers 1a-1c is so designed as to amplify its input signal at the maximum efficiency when the multistage amplifier circuit 1 generates a high power output signal. In other words, an impedance matching circuit (not shown) included in each of the unit amplifier 1a-1c is so constructed that optimum impedance matching is attained at a high power operation of the multistage amplifier 1.
Therefore, when the gain in the initial stage unit amplifier 1a is reduced, a transistor (not shown) operating for power amplification in the initial stage unit amplifier 1a is changed in its equivalent impedance, and the impedance matching with the second stage unit amplifier 1b comes off. The impedance matching between the second and last stage unit amplifiers 1b and 1c comes also off, because the level of the input signal for the second stage unit amplifier 1b is shifted from the optimum input level thereof. Thus, if the power of the output signal SO is reduced by decreasing the DC power supplied to the initial stage unit amplifier 1a, the total power efficiency in the multistage amplifier 1 is reduced.
The decrease in the power efficiency is serious in a portable transceiver in which the DC power source 5 is a battery. This is because the power demand does not decrease in proportion to the decrease of the output signal power, and therefore, the lifetime of the battery is not improved, even if the output signal power is reduced for power saving.